The present invention relates to an apparatus and a process for evaluating the printing quality of a printed product produced on an offset printing machine, in which the printed product and a reference are each divided into image elements and photoelectrically measured, in which for each image element, a respective reference reflectance value R.sub.s and an actual reflectance value R.sub.i are determined and the respective reference and actual reflectance values of corresponding image elements compared with each other, and in which a quality measure Q is determined from those comparisons.
The evaluation of print quality and the regulation of ink feed are usually effected by means of standardized color control strips. These control strips, printed together with the products to be printed, are evaluated densitometrically and the ink color values of the printing machine set accordingly. The measurement of the color control strips may take place on the printing machine while it is running by means of so-called machine densitometers, or off-line, for example by means of an automatic scanning densitometer, wherein the control loop in both cases may be open (quality evaluation) or closed (machine regulation) in relation to the inking systems. A representative example of a computer-controlled printing machine having closed control loop is described in U.S. Pat. Nos. 4,200,932 and 3,835,777, among others.
In actual practice, it very frequently occurs, for example for reasons of format, that the use of a color control strip is not possible. In such cases, the quality is usually evaluated by visual means, as before.
More recently, a system has become known (for example from the published U.K. application No. 2 115 145), making possible the machine evaluation of printed products without using color control strips. In this system, the printed products are scanned photoelectrically over the entire image surface, with the measurement thus being performed "in the image." Measurements are effected on the running printing machine by means of a machine densitometer, by image elements. As an optional process, the scanned values of the individual image elements are compared, after special processing, with the processed scanning values of a reference printed product ("0.K. sheet"), and with the aid of the results of this comparison, a quality decision, either "good" or "poor," is made in compliance with certain decision criteria. The decision criteria include such factors as the number of image elements differing by more than a certain tolerance from the corresponding image elements of the reference, the differences of the scanning values summed over selected areas of the image form the corresponsing scanning values of the reference, and the differences summed over certain scanning tracks of the scanning values from the corresponding values of the reference.
"In the image" measurements are also known, from U.S. Pat. Nos. 3,958,509, EP Publ. No. 29561 and EP Publ. No. 69572, among others. In the systems described therein, the surface coverage of printing plates is determined by zones and evaluated for the purpose of manual or machine presetting of the ink feed control elements of the printing machine. However, this involves a single presetting, and there is no quality evaluation of the printed products.